What is the pH of a salt solution

What is the pH of a salt solution

pH of Salts :
The aqueous solutions of all kinds of salts do not have the same pH value.

(i)   Salts of strong acids and strong bases : Sodium chloride (NaCl), potassium nitrate (KNO3), sodium sulphate (Na2SO4), etc., are salts of this category. The aqueous solutions of these salts are neutral with pH value of 7.

(ii) Salts of strong acids and weak bases : Aluminium chloride (AlCl3), copper sulphate (CuSO4), zinc sulphate (ZnSO4), etc., are salts of this category. The aqueous solutions of these salts are acidic with pH value less than 7.

(iii) Salts of weak acids and strong bases : Sodium acetate (CH3COONa), sodium carbonate (Na2CO3) and sodium hydrogencarbonate (NaHCO3) are examples of this category of salts. The aqueous solutions of these salts are basic in nature with pH value more than 7.

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Role of pH in everyday life

Role of pH in everyday life

Role of pH in everyday life :
pH plays a very important role in our everyday life.

1. In our digestive system : Hydrochloric acid produced in our stomach helps the digestion of food without causing any harm to the stomach. But when the amount of the acid goes beyond a certain limit due to indigestion, pain and irritation are created in the stomach. So, in order to neutralize the effect of excess acid, a mild base called antacid is usually taken. Magnesium hydroxide (milk of magnesia) is a mild base which is usually used as an antacid.

2. Acids cause tooth decay : When we eat sugary food, it gets degraded by bacteria present in the mouth and an acid is formed. When the pH becomes lower than 5.5, tooth enamel gets corroded. Saliva, which is slightly alkaline, produced in the mouth neutralizes some acid, but excess acid remains unaffected. The excess acid can be removed only by the use of toothpaste which is alkaline. Neem stick contains alkaline juice. So, the cleaning of tooth by Neem stick also helps to reduce tooth decay.

3. Acid is produced in fatigued muscle : As a result of physical exercise, stiffness and pain in the muscle starts due to the formation of lactic acid. The supply of oxygen in the muscle is reduced. This causes difficulty in the release of energy leading to increase in the rate of anaerobic metabolism. As a result, lactic acid gets accumulated in the muscles.

4. Some animals and plants contain acids : Honey-bee injects an acid through its stings which causes pain and irritation. Hence, a mild base like baking soda is applied to treat the wound. Similarly, nettle leaves, which have stinging hairs, when touched inject formic acid in our body. This causes a burning pain.

Note : Nettle is a stinging plant. When one accidentally touches its hairs, a painful effect is produced. As a remedy, the affected area is rubbed with the dock plant. The dock plant is alkaline which neutralizes the effect of the acid.

5. The brilliance of a tarnished copper vessel can be restored by using acid : You know, lemon juice contains an acid. In order to clean a copper vessel, we rub it with the piece of a lemon. The tarnish on the vessel is caused by the formation of a layer of basic copper oxide. Since lemon juice contains citric acid, it reacts with the copper oxide to form copper citrate and is washed away. The vessel then regains its shining appearance.

6. pH of soil : Soils are generally acidic. Plants require definite pH range for their proper growth. They do not grow in alkaline soil. Many plants do not grow properly in highly acidic or highly alkaline soil. So, highly acidic soil is treated by spreading quicklime, slaked lime or calcium carbonate to lower its acidity.

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How can we measure the strength of acids and alkalis?

How can we measure the strength of acids and alkalis?

    1. An acid produces hydrogen ions in aqueous solution. The acidity of a solution is a measure of the concentration of the hydrogen ions in the solution.
  1. A base produces hydroxide ions in aqueous solution. The alkalinity of a solution is a measure of the concentration of the hydroxide ions in the solution.
  2. In 1909, Soren Sorensen proposed the pH scale for measuring acidity or alkalinity of an aqueous solution.

The pH scale

How can we measure the strength of acids and alkalis 1

  1. The pH scale normally has a range of values from 0 to 14 to indicate how acidic or alkaline an aqueous solution is.
  2. The pH value measures the concentration of hydrogen ions or hydroxide ions.
    pH < 7 → acidi solution
    pH = 7 → neutral solution
    pH > 7 → alkaline solution
  3. The lower the pH value, the higher the concentration of hydrogen ions.
    The higher the pH value, the higher the concentration of hydroxide ions.
  4. The pH of values some common solutions used in daily life are shown below.
    How can we measure the strength of acids and alkalis 2
  5. The pH value of an aqueous solution can be measured by using
    (a) Universal Indicator
    (b) pH meter
    (c) acid-base indicators
    How can we measure the strength of acids and alkalis 3

Table shows examples of acid-base indicators.

IndicatorpH rangeColour change
Acid         NeutralAlkali
Methyl orange3.0 – 5.0RedOrangeYellow
Bromothymol blue6.0 – 8.0YellowGreenBlue
Phenolphthalein8.0-10.0ColourlessColourlessPink

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Strong and weak acids

  • The strength of an acid or alkali depends on the degree of dissociation of the acid or alkali in water.
  • The degree of dissociation measures the percentage of acid molecules that ionise when dissolved in water.
  • A strong acid is an acid which ionises or dissociates completely in water to produce a high concentration of hydrogen ions.
  • Hydrochloric acid is a strong acid. It is 100% ionised in water. All the hydrogen chloride molecules that dissolve in the water ionise completely into hydrogen ions and chloride ions.
    How can we measure the strength of acids and alkalis 4
  • A weak acid is an acid which ionises partially in water to produce a low concentration of hydrogen ions.
  • Ethanoic acid is a weak acid. Dilute ethanoic acid is about 0.4% ionised, only 4 out of every 1000 ethanoic acid molecules ionise. As fast as the acid molecules ionise to produce ions, the ions combine back again to give the original acid molecules in a reversible reaction.
    How can we measure the strength of acids and alkalis 5
    How can we measure the strength of acids and alkalis 6
  • Examples of strong and weak acids are shown in Table.
    AcidNameParticles
    Strong acidHydrochloric acid, HClH+, Cl
    Nitric acid, HNO3H+, NO3
    Sulphuric acid, H2S04H+, HSO4, SO42-
    Weak acidCarbonic acid, H2C03H+, HCO3, CO32-, H2CO3
    Ethanoic acid, CH3COOHH+, CH3COO, CH3COOH
    Sulphurous acid, H2SO3H+, HSO3, SO32-, H2SO3
  • Strong acid and weak acid are defined as follows.
    A strong acid is completely ionised in water to produce a high concentration of hydrogen ions.
    A weak acid is partially ionised in water to produce a low concentration of hydrogen ions.
  • The pH of an acid solution changes with concentration of the acid. The concentration of the acid affects the concentration of hydrogen ions produced. For example:
    10 mol dm-3 hydrochloric acid: pH = 1
    01 mol dm-3 hydrochloric acid: pH = 2
  • pH values can be used to compare the acid strength of different acids. All the acids to be compared must be of the same concentration.
    For example:
    10 mol dm-3hydrochloric acid: pH = 1
    10 mol dm-3 ethanoic acid: pH = 3
  • For two different acids of the same concentration, the acid with the lower pH value is the stronger acid, i.e. higher degree of ionisation in water.

Strong and weak alkalis

  • Sodium hydroxide is a strong alkali. It ionises fully when dissolved in water.
    How can we measure the strength of acids and alkalis 7
  • A strong alkali is an alkali which is fully ionised in water to produce a high concentration of hydroxide ions.
  • A weak alkali is an alkali which ionises partially in water to produce a low concentration of hydroxide ions.
  • Ammonia is an example of a weak alkali. It is only partly ionised in water, which means the ionisation of ammonia in water is incomplete. Only a small amount of ammonia molecules are ionised in water to produce ammonium ions and hydroxide ions.
    How can we measure the strength of acids and alkalis 8
  • Examples of strong and weak alkalis are shown in Table.
    AlkaliNameParticles
    Strong alkaliSodium hydroxide, NaOHNa+, OH
    Potassium hydroxide, KOHK+, OH
    Barium hydroxide, Ba(OH)2Ba2+, OH
    Weak alkaliAmmonia, NH3NH3, NH4+, OH
    Methylamine, CH3NH2CH3NH2, CH3NH3+, OH
    Hydrazine, N2H4N2H4, N2H5+, OH
  • Strong alkali and weak alkali are defined as follows.
    A strong alkali is completely ionised in water to produce a high concentration of hydroxide ions.
    A weak alkali is partially ionisesed in water to produce a low concentration of hydroxide ions.
  • The pH of an alkali solution changes with concentration of the alkali. The concentration of the alkali affects the concentration of hydroxide ions produced.
    For example:
    10 mol dm-3m-3 sodium hydroxide solution pH = 13
    01 mol dm-3 sodium hydroxide solution pH = 12
  • pH values can be used to compare the strength of different alkalis. All the alkalis to be compared must be of the same concentration. For example:
    10 mol dm-3 sodium hydroxide: pH = 13
    10 mol dm-3 ammonia solution: pH = 11
  • For two different alkalis of the same concentration, the alkali with the higher pH value is the stronger alkali, i.e. higher degree of ionisation in water.